Towable buoy

ABSTRACT

A towable buoy having a tow point for connection to a tow cable of a vessel, wherein the position of the tow point is translationally moveable with respect to the remainder of the buoy. Also, a method of operating the towable buoy.

FIELD OF THE INVENTION

The present invention relates to a towable buoy, particularly, though not exclusively, to a communications buoy to be towed by a submarine.

BACKGROUND OF THE INVENTION

Towed communication buoys have been used for many years to provide communications to submarines. Communications antennae and electronics mounted within the buoy can provide a wide range of frequency coverage. Non-communication type sensors can also be mounted in the buoy. The buoy is physically, electrically and/or optically connected to a submersible vessel (platform) by a retractable tow cable. Modern towed communications buoys need to be operable at a large range of platform speeds and depths and to maintain stability whilst close to the sea surface over a range of sea state conditions.

SUMMARY OF THE INVENTION

According to a first aspect of this invention, there is provided a towable buoy having a tow point for connection to a tow cable of a vessel, wherein the position of the tow point is translationally moveable with respect to the remainder of the buoy.

According to another aspect of this invention, there is provided a method of operating a towable buoy having a tow point which is translationally moveable with respect to the remainder of the buoy, the method comprising towing the buoy behind a moving vessel by attaching a tow cable of the vessel to the tow point, and adjusting the position of the tow point with respect to the remainder of the buoy.

The invention is advantageous in that the tow point adjustment helps to balance hydrodynamic and hydrostatic forces on the buoy. At slow speeds the tow cable orientation is dominated by the positive trim of the buoy resulting in an almost vertical link profile between the vessel and the buoy. At medium to high speed, buoy and tow cable drag takes dominance and so a more horizontal profile of the link results. To improve buoy stability across the operating range, the tow point can be moved such that it can help balance hydrostatic forces on the buoy at low speeds, and hydrodynamic forces at high speeds. As tow speed increases, the offset between the tow point and the buoy centre of gravity is preferably increased both vertically and in the fore/aft direction (horizontally) by translational movement of the tow point with respect to the remainder of the buoy.

Movement of the tow point can be achieved using either a passive or active control mechanism. The passive method could harness changes in tow cable tension and/or drag forces to translate the tow point. The simplest passive method would use a spring to oppose the tow cable tension and locate the tow point accordingly. An active mechanism could use a control loop system to translate the tow point position according to the submarine's speed and or tow cable tension levels, with the tow point movement being under the action of a servomotor, for example.

It is intended that the buoy is stable in pitch and roll when towed a few metres, typically less than 10 m, below the sea surface by a submerged vessel at a wide range of speeds and depths, in sea states ranging from 0 to at least 6. More particularly, the buoy may pitch only slightly (typically less than +/−10 degrees at sea state 6 at 6 m/s) in response to wave perturbations.

To achieve the required stability, the buoy may have a significant offset between its centre of gravity (CoG) and centre of buoyancy (CoB), i.e. a significant metacentric height. The buoy may further comprise means to vary the centre of mass and/or buoyancy of the buoy. The significant metacentric height may be achieved using a keel for the buoy. The keel may be moveable. In particular, the keel may be movable between a deployed position and a retracted position such that, when retracted, the buoy has a small space requirement for stowage. The keel may also be moved in its deployed position so as to alter the centre of mass as required. The buoy may also have an internal ballast tank adjustable to alter the hydrostatic characteristics of the buoy. The buoy may further include a lifting hydrofoil for increasing the buoyancy of the buoy. The hydrofoil may be fixed, or may be actively controlled so as to alter its shape, surface area and/or pitch angle for balancing the buoy in trim as the towing vessel speed and/or depth changes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIGS. 1 and 2 illustrate side and front elevations of a first example of a buoy of this invention;

FIG. 3 illustrates the orbital particle motion that occurs within wave structures;

FIG. 4 illustrates a buoy performance simulation setup;

FIG. 5 a illustrates a buoy depth profile and FIG. 5 b a pitch profile for a 6 m/s tow at SS6;

FIG. 6 a illustrates a buoy depth profile and FIG. 6 b a pitch profile for a 3 m/s tow at SS6;

FIG. 7 a illustrates a buoy depth profile and FIG. 7 b a pitch profile for a 0 m/s tow at SS6;

FIG. 8 a illustrates a buoy depth profile and FIG. 8 b a pitch profile for a 6 m/s tow at SS3;

FIG. 9 a illustrates a buoy depth profile and FIG. 9 b a pitch profile for a 3 m/s tow at SS3;

FIG. 10 a illustrates a buoy depth profile and FIG. 10 b a pitch profile for a 0 m/s tow at SS3;

FIG. 11 illustrates the depth keeping performance of the buoy;

FIG. 12 illustrates a side elevation of a second example of a buoy of this invention; and

FIGS. 13 a to 13 d illustrate various elevations of a third example of a buoy of this invention.

DETAILED DESCRIPTION OF EMBODIMENTS Buoy Set-Up

A first example of a buoy is shown schematically in FIGS. 1 and 2. The buoy 1 has a hydrodynamic body 2, a hydrofoil 3, and a moveable tow point 4. The buoy 1 houses a payload, an adjustable ballast tank 5, a tow point actuator and a hydrofoil actuator. The tow point actuator is operable to move the tow point 4 between a high, forward position for high speed tow, and a low, aft position for low speed tow. The hydrofoil actuator is operable to adjust the shape, surface area and/or pitch angle of the hydrofoil 3. The adjustable ballast tank may include a chamber having an opening to the surrounding water, the volume of the tank being adjustable by moving the position of a piston within the chamber such that the tank contains a variable mass of ballast water. The remainder of the chamber may be filled with compressible gas. The buoy is connectable to a towing vessel by a tow cable (not shown) attached to the tow point 4.

The buoy is configured as follows:

-   -   Tow Point Translation—acting to balance hydrodynamic and         hydrostatic forces. At slow speeds the tow cable orientation is         dominated by the positive trim of the buoy resulting in an         almost vertical link profile. At medium to high speed, buoy and         cable drag takes dominance and so a more horizontal profile of         the link results. This can be achieved using either a passive or         active control mechanism. The passive method could harness         changes in tow cable tension and or drag forces to translate the         tow point. A simple passive method could use a spring to oppose         the tow cable tension and locate the tow point accordingly. An         active mechanism could use a control loop system to translate         the tow point position according to the submarine's speed and or         tow cable tension levels.     -   Buoy Buoyancy—Nett buoyancy (buoyancy subtract mass) is varied         by changing the mass alone, through the ballast tank. This is         important for low tow speeds when the buoy function has to         support variable vessel depths and thus changes in tow cable         mass (assuming tow cable is not neutrally buoyant).     -   Metacentric Height—directly affecting roll and pitch stability.         This is important when maintaining control in the dynamic wave         zone. Stability improves with increasing vertical separation of         CoG and CoB.

Orbital Particle Motion

Orbital particle motion is the movement of particles in the water column below a passing surface wave. Its effect on the buoy 1 will be apparent from the following sections, and a brief introduction to this motion will now be described. As each wave passes along the water's surface the particles in the water column below complete an orbital path; moving upwards on the leading edge of the wave, then horizontally in the direction of the wave at the top, then downwards at the back edge of the wave. In deep water this effect is experienced for many meters below the surface, eventually decaying to less than 5% at a depth approximately half the wavelength. An illustration of the particle movement is shown in FIG. 3 along with a graph of the typical orbital velocities that occur in sea state 6 at a depth 5 m below the mean surface elevation. As can be seen, this orbital particle motion does include a significant vertical and horizontal velocity.

Buoy Performance

To evaluate the ability of the buoy 1 to maintain a shallow depth across the operational envelope a series of simulations were performed, using the setup shown in FIG. 4.

Performance of the buoy 1 has been evaluated for 0, 3 and 6 m/s submarine speeds (V) and in sea states (SS) 3 and 6. Submarine depth (D) was maintained at 102 m. The link length (l) was varied according to the submarine speed and sea state conditions.

The following two sections present the results of the two sea state conditions evaluated.

Sea State 6

To maintain a depth of less than 3 m means that the buoy must attain a position that will be above the trough of many of the larger waves. Extracts of the resulting buoy depth and pitch profiles from the 6 m/s tow in sea state 6 are shown in FIGS. 5 a and 5 b, respectively. The vertical translation of the buoy is generated purely by the orbital motion of the water particles.

This helps to minimise depth variation of the buoy as it passes beneath the waves. The buoy however will react to the localised particle flows at a slower rate due to its size and the tow cable constraint, and so relative depth will change according to the wave height and period.

Buoy pitch was limited to ±15° and was generated by the variable particle vertical velocity acting across the length of the buoy and variations in tow cable tension. The stability of the buoy is very good, with the relatively small variations in both pitch and depth being directly linked to wave motion.

At 3 m/s the buoy still attains vertical motion, caused by the wave particle motion but the increasing depth motion, occurring at the back of the waves, appears to have a larger influence than that driving it up. When each wave passes over, the buoy gets pushed below the trough of the wave and it takes several seconds for the buoyancy to bring the buoy back up to the optimum depth.

The effect of the down-flow being more influential than the up-flow is also evident during the 0 m/s case (see FIGS. 7 a and 7 b). Here the limited tow cable prevents the buoy from climbing above 2 m depth and so the advantage of the up-flow is lost. The effect of the down-flow however is not restricted and so the buoy experiences this for a longer time period.

Sea State 3

With its much smaller wave elevations, it is not necessary for the buoy to enter the wave structure so the buoy maintains a much safer distance from the surface with little chance of broaching.

FIGS. 8 a and 8 b (6 m/s case), FIGS. 9 a and 9 b (3 m/s case), and FIGS. 10 a and 10 b (0 m/s case) show the reduced effect of the particle motion upon both the depth and pitch of the buoy at sea state 3. With the slight adjustments to trim and tow point position the buoy is able to maintain depth at all speeds evaluated.

Tow Cable Study

The submarine pulls the tow cable and buoy through the water but it is up to the buoy to lift the tow cable towards the sea surface. The natural lie of a cable being pulled through the water is directly behind the submarine due to the drag forces acting on it (depending on the tow cable trim).

To achieve the same climb height (H) for a fixed length (350 m) of tow cable at a tow speed of 6 m/s requires as much as four times more lift than at 3 m/s. This is tow cable length dependent, reducing to a factor of three for longer tow cables.

To double the height (H) of the buoy from 100 m to 200 m without changing submarine speed or cable length (350 m) requires an increase in lift demand by a factor of four. This reduces slightly to a factor of three for longer tow cables.

Tow cable weight has a considerable impact on the amount of lift needed to raise it. The size of the impact is speed and tow cable length dependent with slow long tows being the most sensitive.

Impact of Tow Cable on Buoy

Assuming a typical tow cable having a diameter of 0.02 m, mass in air of 0.962 kg/m, CD Axial of 0.015 and CD Normal of 0.4, the drag forces generated in relation to the tow cable are far greater than the drag forces of the buoy alone. The performance of the buoy will be therefore dominated by the necessity to lift the tow cable and so in almost all cases except very low speeds (0 to 0.5 knots) the buoy stability will be dependent upon three forces, namely the buoy drag, lift and tow cable tension.

The ability of the towed buoy system to deliver the payload into its operating zone is dependent upon the buoy being able to generate the large lift forces needed to control the tow cable. The two most common methods of generating significant lift are through water displacement (buoyancy) and dynamic lift using hydrofoils.

Buoy Lift

The lift coefficient of a hydrofoil is dependent upon its angle of attack with respect to the approaching water. Small changes in the angle of attack can result in large changes in the lift coefficient resulting in rapid changes to the buoy's depth. A buoy that utilizes the lift generated by a hydrofoil needs to be stable in pitch to prevent rapid changes in depth.

The buoyancy approach removes the dependency upon through water speed and angles of attack, ensuring a constant lift force is available to hold the tow cable high in the water.

The hydrofoil on the buoy ensures it is capable of generating the lift required to remain above the submarine. To maintain a relatively constant lift coefficient the stability of the buoy in pitch needs to be high. An increase in stability can be achieved by offsetting the hydrofoil generated forces and the tow cable connection point on the buoy. For small angles of attack the variation in drag produced by the hydrofoil are virtually negligible since drag is less than about 2% of the lift generated force.

Instead of pitch generating lift fluctuations, these are caused by changes in the buoy's through water speed if the pitch is held approximately constant as the buoy travels in a passing wave. The orbital particle motion created by the waves interacts with the hydrofoil (hydroplane). The hydrofoil lift reduces as the buoy passes through the trailing section of the wave and so the tow cable tension pulls the buoy down. As the buoy enters the next wave the hydrofoil lift increases again pulling the buoy and tow cable up into the wave structure.

The buoy pitch stability is improved by providing a significant vertical offset between the buoy CoG and CoB. A mean nose up pitch orientation of around 8 degrees generates the required lift. The buoy pitch varies by around +/−8 degrees over a five minute period, well within the optimum lift range of the NACA0012 hydrofoil used in the study. This ensures the changes in the drag coefficient of the hydrofoil remain negligibly small.

Optimized depth performance for a 3 m/s tow speed at 100 m depth is achieved with 150 m of tow cable deployed and with 50 kg of displacement buoyancy assigned to the buoy by reducing its mass.

FIG. 11 shows the depth performance in sea states 3, 5 and 6, with all achieving at least 85% of the simulation period at less than 3 m depth without broaching the surface.

Tow Speed

At 0 m/s the hydrofoil lift is limited to that generated by the particle orbital motion and the movement of the buoy as it is pushed around in the waves. This is unreliable and so additional buoyancy has to be introduced to support the tow cable. In low sea states (0 to 4), the buoy attains a compliant depth performance. In sea states 5 and 6 the performance is less reliable and there is an increased risk of surface broaches.

At 6 m/s the buoy is less stable than at lower speeds. By increasing the vertical and horizontal offset between the hydrofoil and tow cable termination positions, stability improves sufficiently. For the 100 m depth case the tow cable length has to be increased to 230 m. Depth performance with this revised buoy/tow cable configuration is consistent with that at 3 m/s.

Trim Control

The dynamic systems for inclusion in the buoy design to trim the buoy's performance are:

Moving Buoy Tow Point

At low speeds the metacentric forces act to stabilize the buoy in roll and pitch but as speed increases the hydrodynamic forces dominate and any stability provided by the buoyancy and mass becomes negligible. It is therefore important that the tow point can be moved to balance hydrodynamic forces at high speeds and hydrostatic forces at low speeds to improve stability across the operating range. As tow speed increases, the offset between the tow point and the CoG increases both vertically and horizontally. This could be controlled passively by harnessing the changes in the tension or drag forces, or alternatively, actively by means of an electrically powered servo motor as part of a closed loop controlled system.

Variable Buoyancy

Buoyancy is crucial for very low submarine speeds when the hydrodynamic lift of the buoy is insignificant. The mass of the tow cable will have a large effect upon the buoy performance, and this will change according to the submarine depth. The depth keeping performance of the buoy is dependent on buoyancy across the operating speed range, therefore some variation in the buoyancy of the buoy will be required. The amount of buoyancy variation required is tow cable mass dependent with the slow deep submarine case being most reliant.

Active Hydrofoil

The depth keeping requirements for the buoy can be met with a fixed hydrofoil which reduces the complexity of the buoy. However, an active hydrofoil capable of increasing its surface area and/or varying its pitch angle may be used.

Having the ability to make these adjustments will not only widen the operational range of the system but also provide additional flexibility for operations such as surfacing, deployment and recovery, ensuring minimal risk of damage to the buoy and towing platform.

Moving Keel

The stability of the buoy can be enhanced for high speed operation by increasing the separation between CoB and CoG by use of a moveable keel. The moveable keel can be retracted to minimize the height of the buoy for storage within the vessel bay.

Control

The buoy contains actuators to tune tow-point location, ballast/buoyancy, hydrofoil configuration and keel position, but these are only used to tune the buoy hydrodynamic characteristics for the specific speed and depth of operation. As the boat speed and depth do not vary continuously, the actuators on the buoy are only required to change position occasionally or slowly.

There are significant advantages to this approach to controlling the buoy. Firstly, the buoy has a minimum number of closed-loop control systems and no fast acting actuators meaning it is much simpler to design, test and prove than a closed-loop control system. The slow response of the actuators also reduces their power requirements and allows them to be made smaller. The position of each actuator can be predefined for the current boat speed and boat depth, with these values being determined from modeling and tank trials. This significantly reduces the modeling and test time required to ensure that a complex closed-loop control system is stable under all conditions, and replaces it with the requirement to prove a number of fixed configurations of buoy for the differing operating conditions.

The fine control of the buoy depth is performed on a closed loop basis by adjusting the length of tow-cable deployed.

Buoy Design Mechanical

As shown in FIGS. 1 and 2, the buoy 1 has a streamlined body 2 with a hydrofoil 3. Onboard actuators (passive and or active) control the buoyancy, the hydrofoil length, angle of attack and the tow-point location. The buoy may be manufactured from composite materials with static buoyancy provided by sections of high density foam.

The payload electronics are contained within pressure vessels, with the actuators mounted externally from the electronics.

Actuator Control

Control of active actuators in the buoy will be commanded from the towing vessel, with the actuators moving to predefined positions for the specific speed and depth of the vessel.

At low speeds, the buoyancy will be increased and the tow-point moved aft and down. This acts to stop the buoy pitching up at too great an angle as the natural buoyancy dominates the hydrodynamic lift of the buoy. At higher speeds, the buoyancy can be reduced as the hydrofoil provides all the lift required, and a forward located tow-point provides stability.

Additionally, as the depth of the boat increases, the lift required from the buoy will need to be increased to overcome the increased drag and weight of the longer tow-cable and so the hydrofoil and buoyancy need to be adjusted accordingly.

Alternative Buoy Designs

FIG. 12 illustrates a side elevation of a second example of a buoy of this invention. The buoy 101 has a hydrofoil body 102, a moveable keel 103, and a moveable tow point 104. The body 102 houses a payload, an adjustable ballast tank 105, a tow point actuator, and an actuator for moving the keel 103.

The tow point actuator is operable to move the tow point 104 between a high, forward position for high speed tow, and a low, aft position for low speed tow. The tow point actuator includes a wheel 104 a, which can rotate to move the tow point between these two positions. The wheel 104 a is rotatably mounted on the forward end of the body 102. This contrasts with the linear movement of the tow point 4 in the buoy 1 of the first example described previously. However, the tow points 4 and 104 share the common feature of enabling translational movement of the tow point with respect to the remainder of the buoy, under active or passive control. The two types of tow point are interchangeable on the buoys 1 and 101.

The keel actuator is operable to adjust the position of the keel 103. The keel 103 is a heavy, streamlined body of generally circular cross section rotatably mounted on one end of keel arm 103 a, whose other end is rotatably mounted on the underside of the body 102. Under the action of the keel actuator, the keel 103 can be moved between a high, retracted position and a low, deployed position. When in its retracted position, the buoy 101 occupies a smaller space for storage, e.g. onboard the towing vessel. When in its deployed position, the CoG of the buoy 101 is moved aft and downwardly, increasing the metacentric height, and therefore the stability, of the buoy. The position of the deployed keel can be adjusted to vary the position of the buoy CoG. However, it is likely that the keel 103 will be typically fully deployed to maximise the stability of the buoy 101 in pitch and roll. The keel 103 may be used to house control electronics, if desired.

The adjustable ballast tank 105 is similar to the ballast tank 5 of the buoy 1 described previously, and operates similarly.

The buoy 101 is connectable to a towing vessel by a tow cable (not shown) attached to the tow point 104. The buoy may be operated in a similar manner as for the buoy 1 described previously.

FIGS. 13 a to 13 d illustrate isometric, front, back and side elevations of a third example of a buoy of this invention. The buoy 201 has an upper hydrofoil body 202, a pair of lower hydrofoils 203 each mounted from the hydrofoil body 202 by a pair of end plates 203 a, and a moveable tow point 204. The body 202 houses a payload, and a tow point actuator. The tow point actuator may be a linear or rotary actuator similar to that used in the first or second examples of the buoy described previously for moving the position of the tow point 204. An adjustable ballast tank may be disposed in one or more of the upper hydrofoil 202 and the lower hydrofoils 203. The ballast tank(s) may be similar to those used in the first and second examples of the buoy described previously for adjusting the buoyancy of the buoy. One or more actuators may be provided for moving the hydrofoil 202 and/or the hydrofoils 203 for adjusting the lift generated by the buoy under tow. The buoy 201 is connectable to a towing vessel by a tow cable attached to the tow point 204. A tow point guide rail 206 extends in an arc from the upper hydrofoil 202 and between the two lower hydrofoils 203. The end of the tow cable, or tether, (not shown) incorporates a slider block at one end which slidably engages with the rail 206 to follow the contours of the rail according to the local catenary shape. The guide rail 206 acts as a manual means of adjusting the tow point position. A vertical profiled tether will self locate to the lower central section of the rail 206, and more horizontal tether will locate on the higher forward rail section. The buoy may be operated in a similar manner as for the buoys 1 and 101 described previously.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. 

1. A towable buoy comprising a tow point for connection to a tow cable of a vessel, wherein the position of the tow point is translationally moveable with respect to the remainder of the buoy.
 2. A towable buoy according to claim 1, wherein the position of the tow point is moveable in a fore/aft direction and/or vertically with respect to the remainder of the buoy.
 3. A towable buoy according to claim 1, wherein the position of the tow point is selectable based upon the speed and/or depth of the towing vessel.
 4. A towable buoy according to claim 1, further comprising a passive tow point adjustment mechanism for moving the position of the tow point.
 5. A towable buoy according to claim 4, wherein the passive tow point adjustment mechanism is operable to move the position of the tow point according to loads in the tow point.
 6. A towable buoy according to claim 1, further comprising an active tow point adjustment system for moving the position of the tow point.
 7. A towable buoy according to claim 1, comprising a significant metacentric height.
 8. A towable buoy according to claim 1, further comprising means to vary the centre of mass of the buoy.
 9. A towable buoy according to claim 1, further comprising means to vary the buoyancy of the buoy.
 10. A towable buoy according to claim 1, further comprising an internal ballast tank adjustable to alter the hydrostatic characteristics of the buoy.
 11. A towable buoy according to claim 1, further comprising a keel.
 12. A towable buoy according to claim 11, wherein the keel is moveable.
 13. A towable buoy according to claim 1, further comprising a lifting hydrofoil.
 14. A towable buoy according to claim 13, wherein the hydrofoil is adjustable to alter its shape, surface area and/or pitch angle.
 15. A towable buoy according to claim 1, further comprising a control system for adjusting the hydrodynamic and/or hydrostatic characteristics of the buoy.
 16. A towable buoy according to claim 15, wherein the control system is operable to act according to the speed, depth and/or orientation of the towing vessel.
 17. A method of operating a towable buoy having a tow point which is translationally moveable with respect to the remainder of the buoy, the method comprising: towing the buoy behind a moving vessel by attaching a tow cable of the vessel to the tow point; and adjusting the position of the tow point with respect to the remainder of the buoy.
 18. A method according to claim 17, wherein the position of the tow point is adjusted according to the speed and/or depth of the vessel.
 19. A method according to claim 17, wherein the buoy is towed submerged, preferably just beneath surface water waves. 